1. Field of the Invention
This invention relates to an optical information reproducing apparatus for reproducing information by irradiating a recording medium with a laser beam emitted by a semiconductor laser, and more particularly to a control means of an optical information reproducing apparatus for controlling the quantity of a laser beam to be emitted by a semiconductor laser.
2. Related Art Statement
An optical information reproducing apparatus for optically reproducing information uses, for example, an optical disk to serve as a recording medium. The optical disk is categorized into a type on which information is previously recorded and which enables only reproduction of the information to be performed and a type which enables both recording and reproduction of the same.
Ordinary optical disks of the type, to and from which information can be recorded and reproduced, and which are continuous groove type optical disks conforming to the ISO standard are classified into: fully rewritable disks each having a data region which is formed on the overall surface of the disk and which enables information to be recorded and reproduced; and partially embossed disks containing information formed into pits by a molding process performed at the time of manufacturing the disk and including two types of data regions disposed in a mixed manner on one disk such that data regions, which enables only reproduction of information, and data regions to and from which information can be recorded and reproduced.
A plurality of tracks are formed on the recording surface of the optical disk. The data region of the disk is divided into a predetermined number of tracks and each track is divided into a predetermined number of sectors. Each sector has a header region in the leading portion thereof, the header region including information such as a sector start mark, the track number and the sector number in the form of pits formed at the time of manufacturing the disk. A data region is formed in the rear of the header region, the data region being a region on which data will be recorded. The data region of, for example, the partially embossed disk, has a reproduction-only data region on which information in the form of pits are recorded previously and which enables only reproduction of the information similarly to the header region or a recording and reproducing data region (hereinafter also called a "user data region") to and from which information can be rewritten and reproduced by a photomagnetic method or the like.
In a case where the optical disk having the foregoing structure is a photomagnetic-type optical disk, information in the user data region that enables information to be recorded and reproduced is reproduced by using a Kerr effect. The level of a reproduction signal obtained from the user data region is unsatisfactorily low and suffers from a poor C/N ratio.
In order to improve the C/N ratio and to accurately reproduce information, the power of the laser beam to irradiate the medium at the time of reproducing information must be enlarged. However, enlargement of the power will deteriorate the recorded information due to heat of the laser beam emitted to reproduce the information. Therefore, the adequate range allowed for the power for irradiating the user data region with the semiconductor laser beam at the time of reproducing information has been narrowed excessively.
Since the semiconductor laser involves excessive initial dispersion of the individual characteristics and encounters undesirable change in the characteristics due to change in temperature and as the time passes, the quantity of the laser beam emission must be controlled in order to cause the semiconductor laser to stably emit the laser beam with predetermined power.
A variety of structures have been disclosed to serve as a circuit for controlling the quantity of the laser beam to be emitted by the semiconductor laser, and, for example, an output control circuit has been disclosed in Japanese Patent Laid-Open No. 62-281485 that has an arrangement as shown in FIG. 1.
The disclosed semiconductor laser output control circuit comprises: a semiconductor laser 101, a light detection means 102, on which a portion of a laser beam emitted by the semiconductor laser 101 is made incident; an A/D converter 103 for converting an output signal from the light detection means 102 into a digital signal; a CPU 105 for comparing an output value from the A/D converter 103 and an output value read from a memory 104 at predetermined intervals; a D/A converter 106 for converting the result of the comparison performed by the CPU 105 into an analog signal; and a semiconductor laser drive circuit 107 for supplying an electric current, which is in proportion to an output signal from the D/A converter 106, to the semiconductor laser 101.
In the structure shown in FIG. 1, the CPU 105 changes an instructed value of the output from the D/A converter 106 in such a manner that the error between an actual quantity of the laser beam emitted by the semiconductor laser 101, which is the output value from the A/D converter 103, and a predetermined desired quantity of the semiconductor laser beam read from the memory 104 is made to be zero. Thus, the CPU 105 changes the quantity of an electric current to be supplied to the semiconductor laser 101 so that control for causing the semiconductor laser 101 to stably emit the laser beam with a predetermined desired power is performed.
The optical information reproducing apparatus is operated by a method comprising the steps of: irradiating an information recording surface of the medium with a laser beam; and receiving light reflected by the information recording surface by the light detector so that information is reproduced. It has been known that if light reflected by the medium returns to the semiconductor laser in a quantity larger than a predetermined value, the light emission becomes instable and noise increases in the reproduced signal.
Since the information recording and reproducing apparatus adapted to the photomagnetic method detects an information mark recorded on the optical disk as rotations of a polarizing surface, its optical system for recording and reproducing information cannot employ an isolator optical system in which a polarizing beam splitter and a 1/4 wavelength plate are combined with each other to prevent returning of light reflected by the disk to the semiconductor laser. Therefore, in order to overcome the problem caused from returned light, a method disclosed in, for example Japanese Patent Publication No. 59-9086, has been employed which comprises a means that superimposes high frequency components to eliminate the influence of returned light so as to superimpose a high frequency electric current on a drive current for the semiconductor laser at the time of reproducing information so that the interference between emitted semiconductor laser beam and returned light is canceled.
That is, the information recording and reproducing apparatus adapted to the photomagnetic method usually comprises a semiconductor laser output control circuit that has a structure for superimposing a high frequency electric current on the drive current as shown in FIG. 2. The semiconductor laser output control circuit shown in FIG. 2 comprises, in addition to the structure shown in FIG. 1, a high frequency oscillating circuit (HFM) 108 for superimposing a high frequency oscillated electric current on the output electric current from the semiconductor laser drive circuit 107 at the time of reproducing information. Furthermore, the semiconductor laser output control circuit comprises a plurality of D/A converters 109 and 110 for individually controlling the power at the time of reproducing information and the power at the time of recording information.
The semiconductor laser output control circuit shown in FIG. 2 is able to prevent overlap of the emitted laser beam and returned light as shown in FIG. 3A by superimposing a high frequency electric current on the drive current for the semiconductor laser.
However, optical information reproducing apparatuses developed recently tend to comprise a reduced-size and weight pickup unit in order to record and reproduce information at higher speed. Thus, the optical information reproducing apparatus recently uses a pickup unit that is composed of a separated optical system that is separated into: a movable portion comprising an objective lens, an actuator capable of moving the objective lens in the vertical and radial directions and a prism; and a fixed portion composed of an optical portion comprising a semiconductor laser, a collimator lens and a prism and a light detector for extracting a signal from returned light. Although the structure, in which all optical elements are integrally disposed in the movable portion, results in a constant distance from the semiconductor laser to the reflecting surface of the disk, the separated optical system involves change in the distance from the semiconductor laser to the reflecting surface of the disk due to the movement of the movable portion in the radial direction of the disk. Thus, the apparatus of a type comprising the separated optical system encounters a fact that the interference between returned light and the emitted laser beam cannot completely be canceled as shown in FIG. 3B even if the foregoing high frequency superimposing operation is performed. Thus, the quantity of returned light to the semiconductor laser becomes different between a header region in which information is recorded in the form of pits and a user data region that enables information to be recorded and reproduced to and from a land portion thereof having no pit. Therefore, the control for making the electric current to be supplied to the semiconductor laser to be constant cannot completely prevent the change in the quantity of the laser beam to be emitted by the semiconductor laser.
FIG. 4 shows an example of the I-P characteristics (characteristics of the relationship between drive current and quantity of the emitted laser beam) measured in such a manner that an apparatus for making emitted laser beam and returned light to interfere with each other is used to change the quantity of the laser beam returned to the semiconductor laser. The inclination of the I-P characteristics of the semiconductor laser is changed when the quantity of returned light has been changed. If the laser drive current is made to be a constant value of Ic, the quantity of the emitted semiconductor laser increases from P1 to P3 in proportion to the increase in the quantity of returned light. That is, the semiconductor laser sometimes involves the change in the quantity of the emitted laser beam if the quantity of returned light has been changed.
The quantity of returned light from the disk to the semiconductor laser is different between the header region having information in the form of pits and the user data region having no pit and enabling information to be recorded and reproduced. In the header region, the quantity is reduced as compared with the user data region because of the influence of diffraction, while the same is enlarged in the user data region having no pit.
Then, a case will be considered in the circuit having the structure arranged as shown in FIG. 2 such that monitoring of the output from the A/D converter performed by a CPU is stopped, the instructed value of the output from the D/A converter is maintained, that is, the output electric current from the semiconductor laser drive circuit is made to be constant, a laser beam is emitted in a state where the photomagnetic disk is subjected to tracking servo operation, and the quantity of the laser beam emitted by the semiconductor laser is measured in accordance with the output from the light detection means. Since the quantity of the laser beam emitted by the semiconductor laser is, as shown in FIG. 4, changed due to the change in the quantity of light returned to the semiconductor laser in the foregoing case, a phenomenon is sometimes observed, as shown in FIG. 5, that emitted laser beam quantity P from the semiconductor laser is made to be smaller in the header region involving small quantity of returned light than that in the user data region in which a large quantity of light is returned. Although it is preferable that the quantity of the laser beam emitted by the semiconductor laser is the same between the header region and the user data region, there sometimes arises an excessive difference in the quantity of the emitted laser beam between the respective regions of the disk.
The power for the semiconductor laser has been controlled in such a manner that the CPU checks the quantity of the emitted laser beam at predetermined intervals to make the error from the desired quantity to be zero as described above. When the quantity of the laser beam obtained in the header region is checked, an erroneous discrimination can be made that the quantity of the emitted laser beam is too small although no error in the quantity of the emitted laser beam from a desired quantity has been detected in the operation of checking the quantity of the laser beam performed in the user data region. Thus, there arises a problem in that the quantity of the error is calculated.
Therefore, if the quantity of the electric current to be supplied to the semiconductor laser is changed in accordance with the quantity of the error, the quantity of the emitted laser beam is made larger than the desired value after the emitted laser beam has been moved to the user data region though a correct quantity of emission is obtained in the header region. Thus, there is a risk that information recorded by a user deteriorates due to the strong power beam.
On the contrary, a disk of a type in which the quantity of returned light in the header region is larger than that in the user data region encounters a risk that control of the power in accordance with the result of checking of the quantity of the laser beam performed in the header region causes the quantity of the emitted laser beam in the user data region to be smaller than the desired quantity and thus the C/N ratio deteriorates excessively to read the information.
A partially-embossed-type optical disk will now be considered as the optical recording medium of a mixed type in one disk in which are formed a track region having a recording and reproducing data region that enables information to be rewritten and a track region having a reproduction-only data region that stores information in the form of pits at the time of manufacturing the recording medium and that inhibits information to be rewritten. The partially-embossed-type optical disk involves difference in the quantity of light returned to the semiconductor laser between the two regions. Therefore, if the emission quantity is controlled to realize the desired quantity in each region, a problem arises in that the strong power beam deteriorates information recorded by the user or information cannot satisfactorily be read because the power is too small when the pickup is moved between the two regions.